The present invention is directed to hydrogen peroxide decomposer for use in water-cooled nuclear reactors, including boiling water reactors and pressurized water reactors, for the mitigation of corrosion phenomena in such systems.
Steel pressure vessels and piping exposed to high temperature water are prone to corrosion due to oxidation of the various metals therein by oxidizing agents, particularly oxygen, present in the high temperature water. Corrosion of such vessels and piping can lead to a variety of problems, including stress corrosion cracking, crevice corrosion and erosion corrosion, leading to leakage and/or bursting of such vessels and piping.
In nuclear reactors, significant amounts of heat energy is generated by reactor processes occurring in the reactor core. A liquid coolant, typically water, is used to remove heat from the reactor core and facilitate its conversion to a useable form. A reactor vessel is provided to contain the reactor coolant around the reactor core to effect such heat removal. Further, piping is provided to facilitate transport of the coolant to steam generators or turbines, where heat energy is ultimately converted to electricity. The materials used in the construction of nuclear reactor vessels and piping are elected for their ability to withstand rigorous loading, environmental and radiation conditions. Such materials include carbon steel, low alloy steel, stainless steel and nickel-based, cobalt-based and zirconium-based alloys.
Despite careful material selection, corrosion and, particularly, intergranular stress corrosion cracking (or, simply, stress corrosion cracking (SCC)), is a problem in steel pressure vessels and piping used in nuclear reactors. SCC, as used herein, refers to cracking propagated by static or dynamic tensile stressing in combination with corrosion at the crack tip. Unfortunately, the nuclear reactor environment is conducive to both tensile stressing and corrosion.
Nuclear reactor pressure vessels and piping are subject to a variety of stresses. Some are attributable to the high operating pressure required to maintain high temperature water in a liquid state. Stresses also arise due to differences in thermal expansion of the materials of construction. Other sources include residual stresses from welding, cold working, and other metal treatments.
Nuclear reactors are also susceptible to SCC because of the water chemistry environment of its process systems, which is favourably disposed to corrosion. In this respect, the presence of oxidizing agents, such as oxygen, hydrogen peroxide, and various short-lived radicals, which arise from the radiolytic decomposition of high temperature water in boiling water reactors, contribute to SCC.
Hydrogen peroxide is particularly unstable as it has the ability to act as both an oxidizing agent and a reducing agent. Hydrogen peroxide can act as an oxidizing agent, leading to the formation of water according to the following reaction: EQU H.sub.2 O.sub.2 +2H.sup.+ +2e.sup.-.fwdarw.2H.sub.2 O
As a reducing agent, hydrogen peroxide is oxidized to oxygen according to the following reaction: EQU H.sub.2 O.sub.2.fwdarw.O.sub.2 +2H.sup.+ +2e.sup.-
Because of its ability to act as both an oxidizing agent and a reducing agent, hydrogen peroxide is highly unstable and will spontaneously decompose into water and oxygen according to the following reaction: EQU 2H.sub.2 O.sub.2.fwdarw.2H.sub.2 O+O.sub.2
This will happen if aqueous hydrogen peroxide contacts a metallic surface whose electrode potential lies within this region of instability, which is typically the case in the BWR environment.
Stress corrosion cracking is of great concern in boiling water reactors (BWR's) which utilize light water as a means of cooling nuclear reactor cores and extracting heat energy produced by such reactor cores. Stress corrosion cracking causes leakage or bursting of such vessels or piping resulting in the loss of coolant in the reactor core. This compromises the reactor process control, which could have dire consequences.
To mitigate stress corrosion cracking phenomenon in BWR's, it is desirable to reduce the electrochemical corrosion of metal components that are exposed to aqueous fluids. ECP Electrochemical Corrosion Potential is a measure of the thermodynamic tendency for corrosion to occur, and is a fundamental parameter in determining rates of stress corrosion cracking. ECP has been clearly shown to be a primary variable in controlling the susceptibility of metal components to stress corrosion cracking in BWRs. FIG. 1 shows the observed and predicted crack growth rate as a function of ECP for furnace sensitized Type 304 stainless steel at 27.5 MPa in 288.degree. C. water over the range of solution conductivities from 0.1 to 0.3 .mu.S/cm.
For type 304 stainless steel (containing 18-20% Cr, 8-10.5% Ni, and 2% Mn), it is known that if the ECP of such steel exposed to high temperature water at about 288.degree. C. can be reduced to values below -230 mV (Standard Hydrogen Electrode--SHE) (hereinafter the "critical corrosion potential"), the stress corrosion cracking problem of such steel can be greatly reduced. The same generally applies for other types of steels.
A well-known method to reduce the ECP to less than -230 mV.sub.SHE and thereby mitigate SCC of steel pressure vessels and piping in nuclear reactors, is to inject hydrogen gas to the recirculating reactor feedwater. The injected hydrogen gas reduces oxidizing species in the water, such as dissolved oxygen. This has the very desirable benefit of reducing the corrosion potential of the steel vessel or piping carrying such high temperature water.
As illustrated in FIG. 2, ECP of 304SS in 288.degree. C. water increases more rapidly with continued addition of hydrogen peroxide when compared to the ECP values measured at the same levels of oxygen concentration. Further, even with the use of hydrogen gas injection, SCC in BWRs continues to occur at unacceptable rates when hydrogen peroxide is present. This is illustrated in FIG. 3, where stress corrosion cracking is shown to occur in BWRs, even with the addition of hydrogen gas, when 20-30 ppb of hydrogen peroxide is present. This information suggests that the presence of hydrogen peroxide in reactor systems is a significant contributor to stress corrosion cracking of metal components. Moreover, the present practice of injecting hydrogen gas into the process liquid does not appear to completely assist in the decomposition of hydrogen peroxide and therefore does not bring about the concomitant reduction in ECP that is expected.